The present invention generally provides devices, systems, and methods for determining a position of an electromagnetic energy beam. In many embodiments, the invention provides methods and devices for determining positions of pulsed laser beams having wavelengths of less than 200 nm such as the excimer laser beams used in refractive surgery.
Refractive surgery has changed dramatically over the last several years with the introduction and acceptance of refractive laser eye surgery techniques. Laser eye surgery often employs a laser to effect ablative photodecomposition of corneal tissues, thereby resculpting the ocular optics and correcting vision defects. Ultraviolet laser-based systems often direct a pattern of energy pulses on to the cornea in a controlled manner so as to effect a desired change in the corneal surface shape.
While early refractive laser surgery systems employed variable or ablatable masks to provide progressive shaping of the laser beam, most current refractive laser systems controllably deflect the laser beam so as to scan a laser spot over the exposed corneal surface. The laser will often be used to selectively remove stromal tissues from within the cornea, typically after the overlying epithelial tissue has been removed or temporarily displaced in procedures referred to as photorefractive keratectomy (PRK) or laser assisted subepithelial keratomileusis (LASEK) and laser in situ keratomileusis (LASIK), respectively. The size of the laser spot on the cornea may or may not be changed during a refractive procedure, and the laser energy is most often delivered as a series of discrete laser pulses, with each pulse removing a portion of the overall ablation shape.
The use of scanned or “flying spot” laser delivery systems can significantly increase the flexibility of a refractive treatment, particularly for treatment of hyperopic, astigmatic, and irregular refractive errors. However, the use of such movable beam systems can complicate certain aspects of the treatment protocols. For example, to achieve a desired resculpting of the corneal tissue, the treatment beam is scanned or otherwise moved across the eye to a large number of different positions. Movement of the beam may be achieved using motorized scanning mechanisms, including offset lenses, movable mirrors, galvanometric actuators, and the like. To achieve the desired resculpting the position of the scanned laser beam should be controlled and/or monitored accurately. If the beam inadvertently resides at one position for too long, due to a jam or malfunction of the scanning mechanism or control system (for example) the desired tissue ablation pattern may not be achieved. In fact, a jam of the scanning system may jeopardize the success of the surgery and could cause damage to the patient's eye. Malfunctions of a scanning mechanism may not be readily detectable by an observer.
To ensure the safety and accuracy of laser refractive procedures, a variety of motion detector systems for corneal laser delivery systems have been developed. Techniques have also been developed for calibrating laser eye surgery systems, determining the characteristics of a laser beam spot, and the like. Exemplary structures are described in U.S. patent application Ser. No. 10/383,445, in U.S. Pat. No. 6,666,855, and in U.S. patent application Ser. No. 10/760,112, as well as U.S. patent application Ser. No. 10/808,728, the full disclosures of which are incorporated herein by reference. While these and other systems have proven effective and safe for monitoring the position of a laser beam for refractive surgeries, as with many successes, still further improvements would be desirable. More specifically, many techniques for determining a position of the laser beam rely on charge couple devices (CCDs) with associated pixel image processing so as to determine a location of a scanned beam. While generally effective, these systems can be fairly complex and expensive, and may have a resolution that is limited by pixel density. The output from the systems may reflect the digital nature of these pixel-based sensors, and the processing time may increase the amount of data processing capability needed for the system and/or may limit the cycle time for positioned confirmation.
A variety of other fields also make use of systems for monitoring or detecting the position of a laser beam. One structure that has been gaining popularity in recent years is the position sensing diode (PSD). PSDs are silicon photodiodes that provide an analog output directly proportional to the position of a light spot on a surface of the detector. PSDs allow simultaneous monitoring of position and light intensity, with the photo generated current from a PSD being proportional to the position of an incident spot of continuous light when the light is within a wavelength range of the PSD. As relatively simple analog devices, PSDs have advantages over CCDs and other pixel-based devices using image processing to determine a location of a light beam.
Unfortunately, existing PSDs are often limited to measurements of continuous incident light beams having a wavelength of over 200 nm, with responsivity of these structures often benefiting from incident wavelengths of at least 400 nm or more. Hence, despite their potential advantages, PSDs have not been widely used for measuring, monitoring, and confirming positions of pulsed laser beams such as those used in laser refractive eye surgery.
In light of the above, it would be advantageous to provide improved devices, systems, and methods for determining positions of laser and other light energy beams. It would be particularly advantageous if these improvements provided the benefits available from position sensing silicon photo-diodes, but expanded those benefits to laser eye surgery systems and other pulsed light-beam scanning and/or positioning systems.